[petsc-users] How to speed up geometric multigrid

[Mark F. Adams]

Ah yes, I agree, geometric MG with Galerkin is worth trying. 


On Sep 23, 2013, at 3:36 PM, Dave May <dave.mayhem23 at gmail.com> wrote:

> Hey Michele,
> 
> I think Galerkin MG, even with a jump in the coefficient of 10^3, can be made more robust and faster for your problem.
> I'd start by trying different smoothers. Try cheby/(bjacobi or asm), or gmres/(bjacobi or asm). When using cheby, make sure you ask for the spectrum estimate.
> Definitely do more than one application of KSP on each level.
> 
> I don't think you should switch to AMG without first trying more robust smoothers and profiling how much time is spent in the coarse grid solve.
> I solve similar problems (geometry and size of the jump) in connection with variable visocsity stokes problems on structured grids, and I'm yet to see a case with AMG is more robust and or faster than Galerkin MG.
> 
> 
> Cheers,
>   Dave
>  
> 
> 
> On 23 September 2013 20:52, Mark F. Adams <mfadams at lbl.gov> wrote:
> 
> >>
> > The simulation does start with both phases and the geometry is supposed to become more complex as the simulation progresses.
> > But so far the run is stopped before there are significant changes in the shape of the droplet.
> 
> Humm,  not sure why you are seeing degradation then.  I imagine the initial geometry is not grid aligned so I'm not sure why you are seeing degradation in convergence rate.
> 
> > I can give a shot to AMG: which options would you suggest to use.
> 
> -pc_type gamg -pc_gamg_type agg -pc_gamg_agg_nsmooths 1
> 
> > Also, how can I project out the constant from the rhs? Thanks a lot!
> 
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetNullSpace.html#KSPSetNullSpace
> 
> 

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